The invention relates to start-up of a synchronous motor drive in general, and more particularly where it involves a load-commutated inverter (LCI) type of motor drive.
In synchronous motor drives using Load Commutated Inverters (LCI), the torque produced at any instant on the rotor of the synchronous motor depends on which pair of inverter thyristors is gated in relation to the instantaneous rotor position. For any rotor position, there is only one pair of thyristors that produces the highest torque value achievable in the required direction for a given DC link current and a given field current. Gating ON any other non-copolar pair will produce a lower torque, or a torque in the direction opposite to the desired sense of rotation of the motor. In order to determine at any the pair of thyristors that should be ON at start-up, the most straightforward approach has been to use a rotor position sensor and to correlate the inverter thyristor gating sequence with the sensed rotor position. This solution was first used in LCI drives and is still used in applications requiring high torque over a wide speed range. The use of a rotary shaft position sensor, however, is by want of a better solution. There are drawbacks associated with a sensor device which include mounting difficulties on large machines, noise sensitivity, susceptibility to stray shaft emf's and a generally poor reliability in a rough environment.
For a number of applications requiring high torque only at high speed, typical of which is flow control, it has been possible to eliminate the inconvenience of a shaft position sensor. One approach is to infer the rotor position from the phase of the stator terminal voltages as induced by the rotating rotor. Such approach works well at medium and high speed however. At low speed, the magnitude of the induced voltages becomes too small to be useful in determining rotor position. Nevertheless, since for flow control applications the torque required at low speed is much reduced, it is possible with a programmed gating sequence synchronized to a modulation of the DC link current impressed on the LCI to fulfill the requirement of accelerating the motor drive, through the low speed range, until a somewhat higher speed when the stator voltages have become effective to indicate the rotor position. This approach makes it difficult to start the motor from a stalled condition. Since there is no position sensor, the rest position of the rotor is unknown and gating of a thyristor pair at random could result in motion in the wrong direction.
The present invention overcomes the latter problem, in that it avoids a shaft position sensor while insuring in a simple and straightforward fashion starting of the motor drive from any stalled motor position.